MAX17480GTL 데이터 시트보기 (PDF) - Maxim Integrated

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MAX17480GTL

AMD 2-/3-Output Mobile Serial VID Controller

Maxim Integrated 

AMD 2-/3-Output Mobile Serial

VID Controller

select the boost capacitors to avoid discharging the

capacitor more than 200mV while charging the high-

side MOSFETs’ gates:

CBST

=

N × QGATE

200mV

where N is the number of high-side MOSFETs used for

one SMPS, and QGATE is the gate charge specified in the

MOSFET’s data sheet. For example, assume two

IRF7811W n-channel MOSFETs are used on the high

side. According to the manufacturer’s data sheet, a single

IRF7811W has a maximum gate charge of 24nC (VGS =

5V). Using the above equation, the required boost

capacitance would be:

:

CBST

=

2 × 24nC

200mV

= 0.24µF

Selecting the closest standard value, this example

requires a 0.22µF ceramic capacitor.

NB SMPS Design Procedure

NB Inductor Selection

The switching frequency and operating point (% ripple

current or LIR) determine the inductor value as follows:

:

L3

=

⎛

⎝⎜⎜

VIN3 − VOUT3

fSW3ILOAD3(MAX)LIR

⎞⎛

⎠⎟⎟⎝⎜

VOUT3

VIN3

⎞

⎟

⎠

where ILOAD3(MAX) is the maximum current and fSW3 is

the switching frequency of the NB regulator.

Find a low-loss inductor having the lowest possible DC

resistance that fits in the allotted dimensions. If using

a swinging inductor (where the inductance decreases

linearly with increasing current), evaluate the LIR with

properly scaled inductance values. For the selected

inductance value, the actual peak-to-peak inductor rip-

ple current (∆IINDUCTOR) is defined by:

:

∆IINDUCTOR

=

VOUT3 (VIN3 − VOUT3 )

VIN3fSW3L3

Ferrite cores are often the best choice, although pow-

dered iron is inexpensive and can work well at 200kHz.

The core must be large enough not to saturate at the

peak inductor current (IPEAK3):

IPEAK3

=

ILOAD3(MAX)+

⎛

⎝⎜

∆IINDUCTOR

2

⎞

⎠⎟

NB Peak Inductor Current Limit (ILIM3)

The MAX17480 NB regulator overcurrent protection

employs a peak current-sensing algorithm that uses the

high-side MOSFET RON(NH3) as the current-sense ele-

ment. Since the controller limits the peak inductor cur-

rent, the maximum average load current is less than the

peak current-limit threshold by an amount equal to half

the inductor ripple current. Therefore, the maximum

load capability is a function of the current-limit setting,

inductor value, switching frequency, and input-to-out-

put voltage difference. When combined with the output

undervoltage-protection circuit, the system is effectively

protected against excessive overload conditions.

The peak current-limit threshold is set by the ILIM3 pin

setting (see the Offset and Current-Limit Setting for NB

SMPS (ILIM3) section).

NB Output Capacitor Selection

The output filter capacitor must have low enough ESR to

meet output ripple and load-transient requirements. In

CPU VCORE converters and other applications where the

output is subject to large load transients, the output

capacitor’s size typically depends on how much ESR is

needed to prevent the output from dipping too low under a

load transient. Ignoring the sag due to finite capacitance:

:

(RESR

+ RPCB )

≤

VSTEP

∆I LOAD(MAX)

The output capacitor’s size often depends on how

much ESR is needed to maintain an acceptable level of

output ripple voltage. The output ripple voltage of a

step-down controller equals the total inductor ripple

current multiplied by the output capacitor’s ESR. For

single-phase operation, the maximum ESR to meet the

output-ripple-voltage requirement is:

:

RESR

≤

⎡⎣⎢⎢(VIN3V−INV3OfSUWT33L) V3OUT3

⎤

⎦⎥⎥ VRIPPLE3

where fSW3 is the switching frequency. The actual

capacitance value required relates to the physical size

needed to achieve low ESR, as well as to the chemistry

of the capacitor technology. Thus, capacitor selection

is usually limited by ESR and voltage rating rather than

by capacitance value (this is true of polymer types).

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